Photo of Chalcedony in Processed & Rough Form
Chalcedony is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of silicon dioxide, their full chemical compound being SiO2.
Chalcedony is a compact form of silica, composed of microscopic quartz crystals. It is softer than quartz and denser than opal. Its appearance may range from transparent to translucent to opaque. The main types are chalcedony, which is uniformly colored, and agate, which has curved bands or zones of varying color.
Varieties include the red to reddish brown, translucent carnelian, the red, opaque jasper, the bright-green chrysophrase, moss agate with dark, branchlike patterns, and heliotrope, green with red spots. Flint and chert are impure forms of chalcedony.
Chalcedony occurs in some fossils, such as petrified wood. It forms in cavities in rocks of different types, especially lavas, and develops at relatively low temperatures.
Visible examples would be the agate in Rio Grande de Sul (Brazil), carnelian in Uruguay and California, chrysoprase in Queensland (Australia), and flint in the chalk of the white cliffs of Dover (England).
The specific gravity [?] for Chalcedony is 2.61, its refractive index [?] is 1.53-1.54, and its double refraction [?] is 0.004.
History
The name chalcedony probably comes from Calcedon or Chalcedon, an ancient port on the Sea of Marmara, in Asia Minor. The Greek khalkedon and Latin charcedonia do not appear, at least from the description of Pliny the Elder, to be the same mineral as the modern chalcedony. Its abundance and durability has made it one of the earliest materials used by man to make pointed tools, knives, and even food containers.
Industrial Usages
Chalcedony has been used since time immemorial as gems, because of their color, hardness, and ability to take a good polish, and as precious materials for the production of ornaments or small sculptures.
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Agate Chalcedony: Agate Chalcedony differs from other Chalcedony in that it often has distinct banding. In fact some slices of Agate may appear to have banding similar to that of tree rings. Due to its porous nature it is often colored (by humans) black, red, green, or blue to further increase its visual appeal. Agate Chalcedony is found in Brazil, China, Germany ,Hungary, Mexico, Uruguay, the United States, and (read full)
Chrysoprase: Chrysoprase is a variety of chalcedony, usually black or leek-green in color. The most highly-prized variation comes in bright-green, apple- or leek-green. It has a waxy lustre. It occurs as mammillary or botryoidal masses. The microscopic quartz fibers have a radial structure. The pigment is nickel. Large broken pieces are often full of fissures with irregular colors. Its color can fade in sun (read full)
Carnelian: Carnelian is a variety of chalcedony, which is translucent and is red to reddish-brown. It may also occur as flesh-colored chalcedony, known as cornelian or carnelian. It is slightly similar in appearance to its relation, the dark brown chalcedony, sard. The colour of cornelian is caused by colloidally dispersed hematite. Other reddish chalcedonies may have been colored by heating since if they (read full)
Sardonyx: Sardonyx is a gem variety of reddish-brown colored sard with white or black banded chalcedony or onyx, used to make cameo with the raised black, red, or brown background. It is an example of a layer stone subvariety of Chalcedony, which in turn is a cryptocrystalline variety of Quartz. Layer stones are multi-layered materials used in the art of gem-carving and engraving, also called glyptography (read full)
Plasma Chalcedony: Plasma is a massive, semitransparent, leek to dark green variety of Chalcedony (a microcrystalline variety of the Quartz group), microgranular or microfibrous, consisting of speckles of red or brownish-red jasper, resembling drops of blood. It is translucent to opaque, may be dark green to bright-green, apple-green, or nearly emerald-green, containing microfibrous of actinolite, frequently fleck (read full)
